For decades, cowboys known as 'tick riders' have patrolled the U.S.-Mexico border to prevent cattle carrying 'fever ticks' from entering.

LAREDO, Texas — The two cowboys knew the Red Angus calf could be trouble as soon as they spied it cowering behind a green patch of mesquite near a bluff north of the Rio Grande.

The calf did not belong here. This land has been closed to cattle for years, and the calf might have crossed in from Mexico where ticks carry a parasite that can kill up to 90% of a herd north of the border.

It was up to the cowboys, federal inspectors known as "tick riders," to capture the calf and figure out where it came from.

About 100 tick riders patrol the Rio Grande in Texas from its mouth in Brownsville to Del Rio, about 380 miles to the west. From there, the land turns too inhospitable for ticks to survive.

The tick riders have been patrolling the U.S.-Mexico border since 1938, their efforts part of a federal tick eradication program started in 1906. The fever tick was declared eradicated on U.S. soil in 1943, and the riders work a quarantine zone established that year. Though the inspectors now use two-way radios and GPS, they still depend on a good horse and a strong rope.

Tick rider Frankie Sullivan and his supervisor had been planning to patrol another part of the border when they stumbled across the calf. Now their plans would have to wait. They pulled their trucks over and prepared to unload their horses.

Sullivan, 54, was "ranch raised" in East Texas and has patrolled for a decade. He has bachelor's and master's degrees in agricultural education from Sam Houston State University and worked in the livestock business until a drought gripped Texas in the 1990s and "the cattle numbers got low." He still wears a rodeo belt he won riding in the town of Anahuac in 1991.

"Sometimes you ride a horse more than you drive a truck," he said.

In his black felt hat and leather chaps, Sullivan strode into the brush, spurs strapped to his dusty boots. It was a hot, sunny day. He bent to scan the bushes, then the dusty gravel ranch road winding south. He was looking for tracks, "cutting for sign," as tick riders say.

A tick rider can read history in the ground. On these scrubby hills, they often divine the movements of cattle, deer, migrants and smugglers. But not today.

"Nothing," Sullivan said.

He began unloading his truck trailer. Out came a red roan colt named Banjo and Sullivan's herding dog, a blackmouth cur he calls Tug Boat.

Supervisor Bill Coble saddled up on a gray quarter horse named Captain and wore a ball cap over his gray hair and matching mustache. Sullivan released the dog and joined him.

The two didn't need to talk much. Sullivan usually rides the trail alone, with occasional visits from Coble, 60, whose belt buckle says "USDA Tick Roping 1982," a trophy he won at the annual tick rider rodeo.

When Coble was young, he worked at the feedlot his father ran in Laredo. After high school, he left to study animal science at Sul Ross State University in West Texas, but dropped out at age 20 to work for the government as a tick rider.

"It is the cowboy life," he said. "It's just you answer to the government, not a ranch manager."

Tug Boat bayed the calf up, cornering it against a fence. Coble circled, then swung his lasso high against the cloudless sky, roping and dragging the stubborn Red Angus to the trailer.

After it was locked inside, the cowboys inspected, or "scratched," it.

No ticks. No brand or markings, either.

In Mexico, where the wasting disease is endemic, cattle have developed immunity, but "Texas fever" can be lethal to cattle on the U.S. side of the border. Surviving cattle display a nervousness known as "tick poverty" or "tick worry." Infected cattle are quarantined but not destroyed.

This calf, a male about 4 months old and 300 pounds, seemed healthy. Even though it can take weeks for cattle to show symptoms after they're bitten, the calf had no signs of tick-borne disease, such as high fever, jaundice, convulsions and bloody urine, or "red water."

But where had it come from?

As if in answer to their question, the cowboys heard a lowing down below, from the fertile river plain South Texans call "the vega." It sounded like a mother calling its calf.

"You want to go down yonder?" Sullivan asked.

They walked to the edge of a bluff, past a swarm of tumblebugs, checking the dusty ground for rattlesnakes. Potentially worse dangers lurked closer to the river.

The riders have been unnerved in recent years by the randomness of drug cartel violence in Mexico. The cartels have been waging a bloody turf battle just across the border in Nuevo Laredo, the northern tip of an area called the "Triangle of Death."

Tick riders make a point of not talking about their work around Laredo — you never know who might be on the cartel payroll. While on patrol, they try to stay out of sight. Yet, now Coble and Sullivan were about to ride down to the border with little cover in search of a cow.